It is common practice to blend materials such as mica, talc, kaolin, precipitated calcium carbonate, precipitated silica, fumed silica, barite, zinc oxide, carbon black, etc. into elastomeric, thermoset, and thermoplastic polymers. Inorganic fillers are added as high as 40 to 50 weight percent. The addition of minerals to polymers can improve properties such as strength, stiffness, temperature and impact resistance, dimensional stability, and scratch resistance. In conventional mineral/polymer composite materials, the mineral phases are dispersed within the polymer matrix at the micrometer scale.
Much interest has been created by the more recent advance of producing nanocomposites. Nanocomposites—nanometer sized dispersions of organophilic clays in polymers to form polymeric hybrids—have been demonstrated to produce dramatic improvements in mechanical properties, heat resistance, thermal stability, and reduced gas permeability of the base polymer without loss of impact strength. Due to their enhanced barrier properties and clarity, nanocomposites are well suited for use as gas transport barriers in packaging applications. Examples include nylon-based nanocomposites for food and beverage packaging which incorporate the nanocomposite layer within single or multi-layer films. Reduction in gas diffusion is attributed to the presence of the clay particles which act to increase diffusion path length. Current nanocomposites characteristically contain small amounts of phyllosilicates dispersed in the base polymer, typically six percent or less, producing overall improvements in reduction of gas transfer that can be calculated from simple diffusion theory and which depend on the generation of a tortuous diffusion path originating from the presence of the dispersed organoclay. A major impediment to the commercial development of nanocomposites has been the difficulty of producing homogenous dispersions of organoclays within the polymer matrix. To improve the affinity between the hydrophilic clay surface and organic polymers, clays are treated by cation exchange with high-molecular-weight onium salts (e.g., ammonium, phosphonium, and sulfonium). However, even with surface treatment, phyllosilicates can still only be dispersed at the nanoscale into polymers that contain polar functional groups. The presence of these polar functional groups makes high barrier polymers, such as PET, EVOH, and Nylon, sensitive to water, thus requiring their use as multilayer laminates which contain an external, water-barrier layer. The requirement of multilayer laminates thus increases manufacturing costs of flexible packaging films.
Accordingly, there is a continuing need to provide low cost materials which provide superior barriers against gas transport and diffusion.